1. Field of the Invention
The invention relates to an electromagnetic clutch unit that connects a pair of rotary members, which are rotatable relative to each other, to each other such that torque is transmitted therebetween, a four-wheel-drive vehicle, a control method for the electromagnetic clutch unit, and a control method for the four-wheel-drive vehicle.
2. Description of Related Art
There has been a four-wheel-drive vehicle configured such that driving force generated by an engine, which serves as a drive source, is constantly transmitted to front wheels and the driving force generated by the engine is transmitted to rear wheels depending on a travelling state. Conventionally, in such a four-wheel-drive vehicle, a clutch is disposed on the front wheel side of a propeller shaft that transmits the driving force in the longitudinal direction of the vehicle, and another clutch is disposed on the rear wheel side of the propeller shaft (refer to, for example, Japanese Patent Application Publication No. 2004-9954 (JP 2004-9954 A)).
In the four-wheel-drive vehicle described in JP 2004-9954 A, a dog clutch is disposed on the front wheel side of the propeller shaft and a multiple-disc clutch, which transmits the driving force by friction, is disposed on the rear wheel side of the propeller shaft. In the two-wheel-drive mode in which the driving force is transmitted only to the front wheels, transmission of the driving force by the dog clutch and transmission of the driving force by the multiple-disc clutch are both interrupted to stop the rotation of the propeller shaft. Thus, it is possible to reduce the sliding resistance caused by the rotation of the propeller shaft and the stirring resistance of a lubricant caused by a gear connected to the propeller shaft. This leads to a reduction in the specific fuel consumption.
If the rotation of an input-side rotary shaft and the rotation of an output-side rotary shaft are not synchronized with each other, the dog clutch is not able to connect the two rotary shafts to each other such that driving force can be transmitted therebetween. In the four-wheel-drive vehicle described in Japanese Patent Application Publication No. 2004-9954 (JP 2004-9954 A), a control system is configured such that, in order to switch the drive mode from the two-wheel-drive mode to the four-wheel-drive mode, first, the torque of rear wheels generated while the vehicle is travelling is transmitted to the propeller shaft by the multiple-disc clutch to rotate the propeller shaft, the rotations of the two rotary shafts of the dog clutch are synchronized with each other in advance, and then the dog clutch is actuated.
However, for example, in a case where the vehicle is making a turn in the two-wheel-drive mode or a slip occurs in a wheel in the two-wheel-drive mode, a difference is caused in rotational speed between the front wheels and the rear wheels. Therefore, even if the propeller shaft is rotated by actuating the multiple-disc clutch on the rear wheel side, it is not possible to synchronize the rotations of the two rotary shafts of the dog clutch with each other. As a result, when the drive mode should be switched to the four-wheel-drive mode in which the driving force generated by the engine is transmitted also to the rear wheels, there is a possibility that the dog clutch will not be actuated and consequently the drive mode will not be switched to the four-wheel-drive mode.